Conventional vehicles are stopped by means of conventional brakes such as disc brakes or drum brakes which are essentially two surfaces rubbing against each other. When stopping, the mechanical energy of the vehicle is completely converted to heat. Conventional automobiles, buses and trucks currently use such mechanical braking almost exclusively.
Electrical braking has been proposed, (mainly for railroad engines) however, which uses a direct current (DC) generator to slow down and stop a vehicle. These systems often employ an electrical load, such as a resistor, to dissipate the electrical energy as heat, or a battery to store the electrical energy. The electrical load at the output of the DC generator is translated as a mechanical counter torque which is in opposition to the direction of shaft rotation. Electrical resistance is converted to mechanical resistance.
The reverse is true for an electric motor. The greater the mechanical load placed on its rotating shaft (counter-torque), the greater the current that is drawn from the electric power supply. The law of conservation of energy must be obeyed, and there are slight losses as heat for both the generator and the motor.
Examples of electrical braking and battery storage can be found in the following references:
______________________________________ 5,642,023 J. C. Journey June 24, 1997 5,578,911 J. C. Carter, et al. November 26, 1996 5,466,998 S. Kinoshita, et al. November 14, 1995 5,350,985 H. Konrad, et al. September 27, 1994 4,908,553 L. O. Hoppie, et al. March 13, 1990 4,671,577 D. H. Woods June 9, 1987 4,427,928 S. Kuriyama, et al. January 24, 1984 4,330,742 E. Reimers May 18, 1982 4,186,333 M. Kremer January 29, 1980 ______________________________________
Also of possible interest is U.S. Pat. No. 5,428,551 to J. T. Trainor, et al. (issued: Jun. 27, 1995).
In such prior art electric braking DC electric energy often charges the battery. In such systems a problem exists in that when the brake switch is applied and the vehicle starts to slow down (because the battery is an electrical load) generator voltage decreases. When generator voltage drops down to battery voltage, the battery won't charge anymore. The energy capacity (voltage times current times time) is still present in the generator, but it cannot enter the battery, because battery charger voltage must be greater than battery voltage in order to charge it.
The other alternative is to use a generator with a voltage rating much higher than battery rated voltage at the driving speed right before the brakes are applied. The reasoning is that when the vehicle has slowed down to almost a complete stop, generator voltage is still slightly higher than battery voltage, conserving all of the energy into the battery. Unfortunately, this method presents a real physical hazard: extremely high current forced into the battery can severely shorten the battery's life, or it can cause a physically dangerous situation such as a battery explosion.
There is thus a need for a system that prevents the stopping of battery charging at a slow speed (wasting energy) or quickly overcharging the battery (physically dangerous). In other words, there is a need to keep generator voltage slightly higher than battery voltage throughout the braking process.